Such an actuator, also known as a pneumatic actuator, comprises an actuator piston disc that is displaceable in the axial direction between a first position (rest position) and a second position (active/extended position). The displacement is obtained by controlling the supply of a gaseous pressure fluid, such as compressed gas/air, acting against the actuator piston disc. The actuator piston disc act in its turn directly or indirectly against the object that shall be displaced, i.e. the engine valve/gas exchange valve, in order to control the position thereof.
When the actuator piston disc is in the rest position the engine valve is in contact with its seat, and when the actuator piston disc is in the active position the engine valve is open, i.e. located at a distance from its seat. It is known that the actuator in some applications demand for high work pressure/high pressure, for instance in the range 8-30 bar, in order to obtain correct function, i.e. be able to operate together with a combustion engine having a range of revolution amounting to 8-10 thousand revolutions per minute. Thereto it is important in some applications to avoid that the temperature in the actuator and in surrounding details/fluids increases due to the actual operation of the actuator and the accompanying compressor, and this is attained by keeping the pressure ratio at a low level and thereby a so-called increased return pressure is utilized, also know as low pressure/basic pressure. In other words, the pressure of the pressure fluid located downstream the actuator is much higher than the atmospheric pressure, for instance 3-6 bar.
The valve actuator makes use of pneumatics as well as hydraulics for the operation thereof, and in valve actuator solutions the risk of mixing gas and hydraulic liquid is high due to the sealings between the different fluids not being hundred percent fluid tight and after some time it is a risk that the fluids may have negative effect to each other. The great pressure fluid pressures entail that the gaseous pressure fluid, continuously or during pressure peaks, risk to be pressed along the actuator piston rod in the direction towards and into the chamber of the hydraulic circuit, resulting in gas contamination of the hydraulic liquid. A gas contamination of the hydraulic liquid lead to great deterioration of the properties of the hydraulic liquid.